In a vessel of a nuclear reactor of a power station, the fuel assemblies forming the reactor core are periodically inspected and replaced. New fuel assemblies must therefore be loaded into the reactor core and the spent ones removed.
Once the new fuel assemblies are loaded, they need to be checked to verify that they are correctly positioned before the vessel is closed with a cover comprising the upper internals assembly (UIA). The UIA elements comprise projecting pins intended to be introduced into corresponding housings in the fuel assemblies, referred to as “S holes”.
A set of nominal positions is ideally defined for the fuel assemblies. However, it is possible that a fuel assembly will be offset from its nominal position beyond its ability to recenter itself, which creates a risk of the assembly catching when the UIA is lowered. Such an offset can force the entry of the UIA pins into the S holes of the fuel assemblies. Although this does not impact the operation of the nuclear reactor, the fuel assemblies can remain caught on the UIA the next time the vessel is opened, resulting in long and costly maneuvers to free them and creating a potential safety hazard for the facility.
In addition, the fuel assemblies are immersed in water, which makes it even more difficult to free the fuel assemblies in a safe and secure manner.
It is therefore necessary to accurately determine a position for each S hole of the fuel assemblies before lowering the UIA, in order to be able to intervene if one of the assemblies is too far from its nominal position (and is therefore likely to subsequently become stuck on the UIA). Due to various constraints, particularly the fact that the fuel assemblies are immersed in water, it is difficult to accurately estimate the position of the S holes. Turbulence due to variations in the liquid medium (local temperature differences) complicate such estimates.
The position of an S hole is estimated with a certain uncertainty which is the maximum difference between the position “estimated” by the detection method and the “actual” position of the S hole. The actual position of each S hole is not known: it is located within a circle A whose center is the “estimated” position and whose radius is equal to the maximum error of the detection method.
To prevent a fuel assembly from remaining stuck when the cover is opened, a tolerance for the distance that can be allowed between the “nominal” position of the fuel assembly and the actual position is 8.3 millimeters (mm). This is expressed as the “actual” position of the fuel assembly being within a circle B having the “nominal” position of the fuel assembly as its center and having a radius of 8.3 mm.
By incorporating the uncertainty related to the method, this means that the difference between the position estimated by the method, allowing for uncertainty, and the nominal position is less than 8.3 mm.
This condition will be better understood by referring to FIG. 1.
A nominal position 10 of an S hole is known. Circle B is denoted 11 and the radius of the circle 11 corresponds to said tolerance (for example 8.3 mm).
The position estimated by the method is denoted 12. The circle A denoted 13 is centered around the estimated position 12 and has a radius corresponding to the maximum error of the detection method.
To prevent the fuel assembly from catching when the cover is opened, circle 13 must therefore be fully contained within circle 11. Whenever there is a circle 13 that lies at least partially outside of circle 11, the corresponding fuel assembly must be flagged as incorrectly positioned in order to initiate repositioning operations prior to lowering the UTA. These repositioning operations significantly increase the duration of plant shutdown and therefore reduce plant availability.
If the maximum error of the detection method is large, and in particular if it approaches the tolerance value (for example 8.3 mm), the detection becomes ineffective because many assemblies will be considered poorly positioned because of the uncertainty even though their actual position is close to the nominal position.
One major challenge is therefore to minimize as much as possible the maximum error of the detection method, between the position estimated by the method and the actual position of an S hole. The maximum error must be small in any event, given the tolerance for the difference between the actual position and the nominal position.
Several methods are known for estimating the positions of S holes. They generally consist of capturing numerous photos of the vessel from different viewpoints. In these shots, the different S holes are located by image processing methods that can identify circles or ellipses. Then the various shots are positioned relative to each other in order to construct a complete map of the vessel.
However, many sources of error can degrade the accuracy of these estimates, for example:                poor image quality (turbulence due to heat generation, insufficient contrast, etc);        camera instability (variation in angle and height).        
Tests conducted on models where the actual positions of the S holes are known, show that the maximum error between the estimated positions of the S holes and the actual positions can be as large as 7 mm, which is too close to 8.3 mm for these detection methods to be usable.
There is therefore a need to significantly reduce the maximum error associated with prior art detection methods, between the position estimated by these methods and the actual position of the S holes.